Be careful when using this, as users may not expect this functionality.
If there’s only one match for a particular search query, you can save users the hassle of clicking by simply redirecting them to the page/post.
add_action('template_redirect', 'single_result');
function single_result() {
if (is_search()) {
global $wp_query;
if ( $wp_query->post_count == 1 && $wp_query->max_num_pages == 1 ) {
wp_redirect( get_permalink( $wp_query->posts[0]->ID ) );
exit;
}
}
}